The birth of a sibling, especially when the child was between about 2 and 4 years old, was associated with a healthier body mass index (BMI) by first grade, according to the research.

Children the same age who did not have a sibling were nearly three times more likely to be obese by first grade.

"Research suggests that having younger siblings -compared with having older or no siblings - is associated with a lower risk of being overweight," said Julie Lumeng from University of Michigan in the US.

One possible explanation, researchers speculate, could be that parents may change the way they feed their child once a new sibling is born.

With children developing long-lasting eating habits at around three years old, changing dietary habits may have a significant impact.

This study is believed to be the first to track subsequent increases in BMI after a child becomes a big brother or sister.

Researchers also note that children may engage in more 'active play' or less sedentary time in front of screens once a younger sibling is born, contributing to healthier BMIs.